1. Field of the Invention
The present invention relates to an electrode conductor line material for electronic devices such as liquid crystal display devices, and more specifically, to a highly corrosion resistant, low resistance electrode conductor line material with a high thermal stability and little defects such as hillocks and pinholes. The present invention also relates to an electrode conductor line material on which a gate insulation film having a high breakdown voltage is formed by forming an oxidized film by anodic oxidation.
2. Related Art:
Pure metals such as Cu, Al, Mo, Ta, and W, or alloy materials such as Al-Cu, Al-Cu-Si, and Al-Pd have been used as electrode conductor line materials having low electric resistance used in electronic devices. However, in electrode conductor line materials for liquid crystal display devices recently attracting public attention as thin display devices, excellent properties different from those of conventional materials, such as large area conductor line for large screens, highly dense conductor line for high resolution, and array fabrication by a high temperature process, are required. FIG. 1 shows a schematic diagram of a pixel in the array of a thin film transistor (TFT) liquid crystal display device. A pixel opening 1 is provided with a display electrode 2, a gate line 3, a gate electrode 3A, a data line 4, a drain electrode 4A, a source electrode 5, and a TFT active element 6. When the TFT is turned on by the signal on the gate line 3, the potential of the data line 4 becomes equal to the potential of the pixel electrode 2 connected through the source electrode 5. As the result, liquid crystals contained in the upper part of the pixel electrode 2 in the paper direction are oriented making the pixel be in the display condition. Here, an array electrode conductor line material of a liquid crystal display device according to the present invention is used in the gate line 3, the gate electrode 3A, the data line 4, the drain electrode 4A, and the source electrode 5.
The properties required in electrode conductor line materials for liquid crystal display devices include a low electric resistance. A high electric resistance causes various problems such as the delay of signals and the generation of heat, especially in manufacturing large liquid crystal display devices. For this reason, pure Al, which has a low electric resistance, has been used as the conductor line material for liquid crystal display devices. Pure Al excels in etching characteristics, and is a suitable material from the point of view of adhesion with the substrate. However, pure Al has disadvantage that it has a low melting point, and easily causes a defect known as hillock during the heating step in the CVD process after the formation of the conductor line film. This heating step is normally performed at a temperature of about 400.degree. C., and when the conductor line material is observed after this step, defects such as small projections and pinholes may be found. Such small projections are called hillocks and if these occur, the flatness of the conductor line material layer is lost, and oxidized films cannot be formed on the conductor line material layer in subsequent steps. The occurrence of hillocks is a very large problem in the manufacture of liquid display devices. Although the mechanism of the occurrence of hillocks has been unknown, it is considered that hillocks occur when a stress in the compression direction is applied to the thin film by heating due to difference in the coefficient of linear expansion between the thin film and the substrate, and Al atoms are driven by this stress and move along grain boundaries.
The occurrence of hillocks may be prevented by using refractory materials such as Cr, Ti, Ta, and MoTa, because the diffusion of atoms along grain boundaries is unlikely to occur. However, all of these refractory materials have as high specific resistance as 50 .mu..OMEGA..multidot.cm (about 4 .mu..OMEGA..multidot.cm for Al), and their electrical properties are not desirable for conductor line materials. These high resistance materials are not suitable for manufacturing large liquid crystal display devices.
Therefore, the development of Al-based alloy electrode conductor line materials has been tried. Although Al-Cu and Al-Cu-Si alloys were reported in the past, and Al-Ta and Al-Zr alloys were reported recently, these were not satisfactory in both the occurrence of hillocks at a high temperature of 300.degree. C. or above, and high electric resistance.
FIG. 2 shows a cross-sectional structure of a liquid crystal display device. The glass substrate and conductor line materials formed on the undercoat applied on the substrate are protected from dielectric breakdown during the TFT operation by forming a gate insulation film for isolating them from the transparent electrode, the amorphous silicon layer, and the source/drain electrode. The gate insulation film consists of silicon oxide or silicon nitride, and laminated on the conductor line materials. However, the formation of such an oxide or nitride layer consisting of multiple layers requires complicated process steps, and if the layer thus formed has defect, dielectric breakdown may occur.
In order to cope with the complication of the process steps for forming insulation films and the problem of dielectric breakdown, it is desirable to oxidize the electrode conductor line material used in the gate electrode directly by anodic oxidation to form a dense oxidized film, and to replace the above insulation films with such an oxidized film. According to this method, the process for forming the insulation film, which required several steps, may be performed in one step, and the anodic oxidation method is suitable for obtaining dense oxidized films. This is possible when pure Al is used as the conductor line material. Pure Al forms a very dense oxidized film by anodic oxidation. However, as described above, pure Al has the problem of the occurrence of hillocks.